The present invention relates to a method of estimating a communication path formed by a plurality of channels. Thus it relates to a technique referred to as reception diversity, whereby a receiver has a plurality of antennas each associated with a different communication channel. In other words, the invention proposes a method of estimating the impulse responses of the transmission channels.
In a communications system, especially a radio communications system, the receiver receives for each communication channel a signal transmitted by a transmitter. The transmitted signal is subject to amplitude and phase fluctuations in the communication channel with the result that the signal received by the receiver is not identical to the transmitted signal. Signal fluctuations are essentially due to what the skilled person refers to as intersymbol interference. This interference can result from the modulation law employed for transmission and is also caused by multipath propagation in the channel.
It is found that the received signal is generally the result of a large number of reflections in the channel. The various paths taken by the transmitted signal cause various delays at the receiver. The impulse response of the channel represents all such fluctuations affecting the transmitted signal. It is therefore the fundamental characteristic representative of transmission between the transmitter and the receiver.
The impulse response of the channel is used in particular by an equalizer whose precise function is to correct intersymbol interference in the receiver. A standard method of estimating the impulse response consists in placing a training sequence made up of known symbols in the transmitted signal.
The sequence is chosen as a function of the modulation law and the dispersion of the channel. In the present context, “dispersion” is to be understood as meaning the delay affecting a transmitted symbol taking the longest path of the channel relative to the same symbol taking the shortest path. The dispersion is routinely expressed as a multiple of the time between two successive transmitted symbols, i.e. a number of “symbol periods”.
Two examples of prior art techniques for estimating the impulse response of a communication channel are mentioned.
The first technique uses particular training sequences referred to as constant amplitude zero autocorrelation (CAZAC) sequences. These sequences are described in an article by A. MILEWSKI: “Periodic sequences with optimal properties for channel estimation and fast start-up equalization”, IBM Journal of Research and Development, Vol.27, No.5, Sept. 83, pages 426-431.
The GSM cellular mobile radio system uses training sequences TS made up of 26 symbols a0 to a25 taking the value +1 or −1. These sequences have the following properties:                                           ∑                          i              =              5                        20                    ⁢                                           ⁢                      a            i            2                          =        16                                                                                   ∑                          i              =              5                        20                    ⁢                                           ⁢                                    a              i                        ⁢                          a                              i                +                k                                                    =        0                                      if          ⁢                                           ⁢          0                <                            k                          ≤        5            
Letting d denote the dispersion of the channel, which takes the value 4 in GSM, the estimate of the impulse response takes the form of a vector X with five components x0 to x4.
The received symbol sequence S corresponding to the training sequence TS is also made up of 26 symbols, denoted s0 to s25. The natural assumption is made here that the transmitter and the receiver are perfectly synchronized, in which case the estimate of the impulse response X is given by the following expression:       X    k    =                    1        16            ⁢                        ∑                      i            =            5                    20                ⁢                                   ⁢                              a            i                    ⁢                      s                          i              +              k                                ⁢                                           ⁢          for          ⁢                                           ⁢          0                      ≤    k    ≤    4  
The CAZAC technique has the advantage that it is very simple to implement. However, it should be noted that each component of the impulse response is established from only 16 received symbols. Because the training sequence is made up of 26 symbols and the channel dispersion value is 4, there is information in the received signal that is not taken into account and this degrades performance compared to the theoretical ideal.
The second prior art technique uses the least squares criterion. It is described in particular in patent applications FR 2 696 604 and EP 0 564 849. It uses a measurement matrix A constructed from a training sequence TS of length n. The matrix has (n−d) rows and (d+1) columns, where d again represents the dispersion of the channel. The item in the ith row and the jth column is the (d+i−j)th symbol of the training sequence:   A  =      (                   ⁢                                        a            4                                                a            3                                                a            2                                                a            1                                                a            0                                                            a            5                                                a            4                                                a            3                                                a            2                                                a            1                                                            a            6                                                a            5                                                a            4                                                a            3                                                a            2                                                            a            7                                    ⋯                          ⋯                          ⋯                          ⋯                                      ⋯                          ⋯                          ⋯                          ⋯                          ⋯                                      ⋯                          ⋯                          ⋯                          ⋯                          ⋯                                                  a            25                                    ⋯                          ⋯                          ⋯                                      a            21                                ⁢                   )  
The training sequence is chosen so that the matrix AtA, where the operator .t represents transposition, cannot be inverted. This is inherently the case for CAZAC sequences but is also the case for other sequences.
The first four symbols s0 through s3 in the sequence of received symbols are ignored because they also depend on unknown symbols transmitted before the training sequence, given that the value of the channel dispersion is 4. At the risk of using a misnomer, the received signal will therefore be defined as a vector S whose components are the received symbols s4, s5, s6, . . . s25.
The estimate of the impulse response then takes the following form:X=(AtA)−1At.S
This least squares technique is slightly more complex than the preceding technique but it should be noted that the matrix (AtA)−1At is calculated only once. Note also that each component of the estimate of the impulse response X is obtained from 22 received symbols, rather than 16 as in the CAZAC technique. Improved performance can therefore be expected.
However, regardless of the technique used, the impulse response of each channel of the communication path is considered to be independent of the others.
A first object of the present invention is therefore to provide a method of estimating a communication path which takes into account the fact that the various antennas are spatially linked.